When ultrasound is used to treat diseases, owing to the great loss of ultrasonic energy on the transmission path, the ultrasonic intensity focused at a nidus is too low to achieve required clinical therapeutic effect. Therefore, for an ultrasonic treatment apparatus, the tough technical difficulties required to be solved currently are how to reduce severe attenuation of ultrasound on transmission paths as much as possible and how to enhance ultrasonic intensity at treated parts.
In the prior art, a manner for solving the above technical problem is usually obtained by the design of an ultrasonic transducer. For an existing ultrasonic transducer, the size and intensity of the ultrasonic energy focusing area are usually relevant to the emitting area and work frequency of the ultrasonic transducer. The larger the emitting area is, the more the ultrasonic energy focused on the area is; and the higher the work frequency of the ultrasonic transducer is, the shorter the wavelength of the emitted ultrasonic waves is, thus reducing the focusing area and increasing the ultrasonic intensity.
In order to increase the emitting area of the ultrasonic transducer, an ultrasonic transducer is disclosed in US2006/0058678A1, in which ultrasonic sources are fixed on an annular supporting body to increase the emitting area of ultrasonic waves. In order to avoid mutual influence of the ultrasonic sources, the following technical solution is adopted in the design: the ring surface opposite to each ultrasonic source is configured as a notch, therefore, the ultrasonic transducer obtains enhanced focusing gain relative to a transducer with a single ultrasonic source. However, since the notch is provided on the ring surface opposite to the ultrasonic source of the ultrasonic transducer, the effective emitting area of the ultrasonic source on the ring surface is reduced, the notch may cause dispersion of the ultrasonic energy, and the energy of the focusing area of such ultrasound therapeutic applicator serving as an annular integral body is reduced, which is disadvantageous for the enhancement of focusing ability of the ultrasonic transducer. Meanwhile, the technical solution just enlarges the emitting area of the ultrasonic transducer to have a superposition of energy at the focus. When the frequency is relatively low, since the wavelength is relatively long, the focusing ability of the ultrasonic waves is poor and the focusing area is relatively large, thus the ultrasonic intensity of the focusing area is so weak that coagulation necrosis of an target area cannot be formed rapidly and effectively during the ultrasonic therapy. In the ultrasonic therapy of deep tissues of a human body or the like, ultrasonic waves need to pass through human skin, bone tissues, air containing tissues, nerve tissues and the like before reaching a focusing position. If a relatively high frequency is adopted for work, the ultrasound has poor penetrability in tissues, and the above tissues have functions such as absorbing the transmitted ultrasonic waves, which causes reduction and dispersion of energy in the focusing area; and the temperature of tissues will rise after the tissues absorb the ultrasonic waves. When the emitting power of the ultrasonic transducer is very large, the temperature rise of the tissues may cause accidental injuries thereof. Additionally, human tissues have a very big nonlinear effect on the ultrasonic waves, so, if the ultrasonic waves with high intensity are transmitted in human tissues, a large part of the ultrasonic waves will be transformed into higher harmonics of the ultrasonic waves and absorbed by the tissues. At that time, if the ultrasonic emitting power of the ultrasonic transducer is continuously increased, a bigger nonlinear effect will be produced, owing to which the increased ultrasonic energy cannot be effectively transmitted to the expected focusing area, and an acoustic saturation phenomenon occurs, thereby affecting the focusing of the ultrasonic waves.
It can be seen that the above technical problem cannot be effectively solved in the prior art by simply enlarging the emitting area of an ultrasonic transducer and performing superposition of energy.
Actually, the emission and reflection on the opposite surface of the ultrasonic source can be used to enhance focusing gain. For instance, Chinese patent (Publication No.: CN 101140354A) applied for previously by the present applicant discloses a resonant ultrasonic transducer with a resonant cavity comprising an ultrasonic transducer and an ultrasonic reflecting unit which are opposite to each other. Since the ultrasonic reflecting unit is equivalent to an ultrasonic transducer, the resonant cavity is practically formed by two symmetrically disposed ultrasonic transducers. Through resonance of the ultrasonic waves in the resonant cavity, the length of the focusing area of the ultrasonic waves in the direction of the acoustic axis is shorter than that in the case of simply using a single ultrasonic transducer (if two ultrasonic waves with the same frequency come face to face, there will be interference in the area where they meet; when the interference appears, they have the same phase at the central point and have different phases at other points, therefore, the superposition of the two ultrasonic waves will cause distribution away from the center to be weak and the ultrasonic focusing area to shorten), so that the energy is more concentrated and the focusing gain is greatly enhanced. The work mode of the resonant ultrasonic transducer can bring a larger gain to the focusing area of the transducer without increasing the emitting area of the transducer.
Nevertheless, the ultrasonic transducer with such structure has many disadvantages: firstly, the resonant cavity formed by the two transducers is not a sealed annular sphere surface and cannot achieve effective acoustic resonance, and a part of energy may still escape from the opening portion between the two transducers provided opposite to each other, and therefore the ultrasonic energy emitted by the transducers cannot be used sufficiently; secondly, since the two transducers are provided opposite to each other, there is no fixed connection therebetween, which may easily result in the deviation of the two transducers from the resonance condition, so, it should be guaranteed that the ultrasonic path in which the two transducers emit the ultrasonic waves will not be interrupted by other factors, otherwise the desired resonant cavity between the two transducers provided opposite to each other may not be formed, and enough gain cannot be produced in the focusing area, or other focusing areas may be formed to damage other normal tissues; thirdly, the length of the focusing area is compressed only in the direction of the acoustic axis of the resonant cavity, and the lengths of the focusing areas in other directions deviated from the direction of the acoustic axis of the resonant cavity are not compressed, that is to say, the formed focusing area is compressed in its length only in the direction of the acoustic axis of the ultrasonic waves, and the volume of the focusing area is not reduced sufficiently; fourthly, the size of the focusing area of the ultrasonic transducer is still affected by frequency, and the ultrasound has poor tissue penetrability under a work condition of low frequency, so that the technical problem of a great loss of energy on the transmission path cannot be solved; fifthly, the emitting area of the ultrasonic transducer is not large enough.